This invention relates generally to the field of earth borehole logging and, more specifically relates to a method for determining anisotropic properties of subterranean formations surrounding an earth borehole.
It is well known that subterranean formations surrounding an earth borehole can be anisotropic with regard to the conduction of electrical currents. The phenomenon of electrical anisotropy is generally visualized in one of two ways, or a combination thereof, as follows.
In many sedimentary strata, electric current flows more easily in a direction parallel to the bedding planes rather than in a direction perpendicular to them. A reason for this anisotropy is that a great number of mineral crystals possess a flat or elongated shape (e.g. mica and kaolin). At the time they were laid down, they naturally took an orientation parallel to the plane of sedimentation. The interstices in the formations are, therefore, generally parallel to the bedding plane, and the current is able to travel with facility along these interstices which often contain electrically conductive mineralized water. Such electrical anisotropy, sometimes called microscopic anisotropy, is observed mostly in shales.
If a cylindrical sample is cut from a subterranean formation, parallel to the bedding planes, the resistivity of this sample measured with a current flowing along its axis is called the transverse (or horizontal) resistivity R.sub.h. The inverse of R.sub.h is the horizontal conductivity, .sigma..sub.h. If a similar cylinder is cut perpendicular to the bedding plane, the resistivity measured with a current flowing along its axis is called the longitudinal (or vertical) resistivity R.sub.v. The inverse of R.sub.v is the vertical conductivity, .sigma..sub.v. The anisotropy coefficient .alpha., by definition, is equal to: ##EQU1##
Subterranean formations are often made up of a series of relatively thin beds having different lithologic characteristics and, therefore different resistivities. In well logging systems the distances between the electrodes or antennas are great enough that the volume involved in a measurement may include several such thin beds. When individual layers are neither delineated nor resolved by a logging tool, the tool responds to the formation as if it were a macroscopically anisotropic formation. A thinly laminated sand/shale sequence is a particularly important example of a macroscopically anisotropic formation.
In situations where the borehole intersects the formations substantially perpendicular to the bedding planes, conventional induction and propagation well logging tools are sensitive almost exclusively to the horizontal components of the formation resistivity. When the borehole intersects the bedding planes at an angle, a so-called deviated borehole, the tool readings contain an influence from the vertical resistivity as well as the horizontal. This is particularly true when the angle between the borehole and the normal to the bedding planes becomes large, such as in directional or horizontal drilling where angles near 90.degree. are commonly encountered. In these situations, the influence of vertical resistivity can cause discrepancies between measurements taken of the same formation in nearby vertical wells, thereby preventing useful comparison of these measurements. In addition, since reservoir evaluation is typically based upon data obtained from vertical wells, the use of data from wells drilled at high angles may produce erroneous estimates of formation producibility if proper account is not taken of the anisotropy effect.
A number of techniques exist for measuring formation anisotropy and/or vertical conductivity, such as by providing transmitter and/or receiver coils that are perpendicular to the borehole axis in addition to coils having conventional orientations.
Equipment and techniques that determine horizontal and vertical conductivity (or anisotropy) by employing special equipment dedicated specifically to such purpose result in increased equipment cost and increased logging time and/or cost.
The present invention is directed to overcoming, or at least minimizing, one or more of the problems set forth above by providing a new method for determining the anisotropic properties of subterranean formations. This method permits the use of a conventional induction type logging tool to determine such properties.